1. Field of the Invention
The present invention relates to a hard disk drive, and more particularly, to a method of measuring the thermal pole tip protrusion (TPTP) of a magnetic head and controlling a recording current based on the degree of TPTP.
2. Description of the Related Art
A hard disk drive is one type of auxiliary storage device used in a computer, which reads/records data from/to a magnetic layer on a hard disk, using a magnetic head. In general, in a hard disk drive, a recording magnetic head is formed of metal, such as Permalloy containing 80% of Ni and 20% of Fe, and a slider, which supports the head, is formed of a non-metallic material.
During a write operation, a Joule of heat is generated when a recording current flows through a metal recording coil and causes a difference between coefficients of thermal expansion of the recording magnetic head, which is a metal material, and the slider which is a non-metallic material. The difference in thermal expansion causes the circumference of a pole of the head to protrude. Such a phenomenon is called thermal pole tip protrusion (TPTP).
TPTP reduces the margin of a head/disk interface (hereinafter referred to as “HDI”), resulting in a decrease in the flying height of the head. If TPTP is excessive, the HDI deteriorates. In a worst-case scenario, excessive TPTP causes a collision of the disk with the head, damage to the head pole, and thermal asperity (TA).
The degree of TPTP is proportional to i2R. Here, i denotes a recording current flowing through a recording coil, and R denotes the resistance of the recording coil. To reduce the degree of TPTP, the recording current i and resistance R need to be reduced. In detail, the resistance R is a factor determined by the properties of the recording coil and the value of the resistance R is fixed during manufacture of the head. The recording current i is determined by a write current (WC) or an overshoot current (OSC) used by the hard disk drive. Because, the degree of TPTP is proportional to the square value of the recording current i, that is, TPTP is affected more greatly by the recording current i than the resistance R.
Since hard disk drives have been designed to be highly integrated, the flying heights of heads have become lower. Thus, problems caused by TPTP cannot be ignored. In this connection, Japanese Laid-Open Patent Publication No. 2001-229637, published on Aug. 24, 2001, discloses a method of determining whether the flying height of a head falls within a desired level, based on a gain of a servo signal.
The problems caused by TPTP are well known but a method of precisely measuring the degree of TPTP has yet to be designed. Therefore, controlling hard disk drives by applying the same TPTP value to their heads is unavoidable. However, because every head has its own TPTP degree, it is not effective in all cases to apply the same TPTP value to different heads, and thus, it is difficult to effectively control hard disk drives.
However, the degree of TPTP of a head is not considered or is considered standard during conventional hard disk optimization. Thus, it is difficult to effectively optimize a write channel of a hard disk drive.